The present invention relates to a device for roll-sealing sheets, especially a formed bottom sheet comprising small pockets filled with pharmaceutical products, to a lidding sheet by means of heat-sealing in a so-called thermoforming machine. The formed bottom sheet and the lidding sheet are conducted between a feed roll and a sealing roll, which presses with a sealing force against the feed roll and thus seals the sheets together to form a sealed composite sheet structure.
To be able to process all types of lidding sheets at high web speeds during a continuous roll-sealing operation with the use of a feed roll and a heatable sealing roll, it is necessary to install a preheating unit upstream of the actual sealing operation. The use of a preheating step has the effect of increasing the time available for heating the lidding sheet to the sealing temperature. The lidding sheet usually consists of aluminum foil coated with a heat-seal lacquer. Lidding sheets in which, in addition, the outside surface of the aluminum foil is laminated to a layer of paper are being used increasingly for child-proof blister packs. Because paper is a poor conductor of heat, however, achieving a high processing output requires a longer preheating time to heat the sealing lacquer to the melting point before the lidding sheet enters the sealing station.
None of the preheating units used in conjunction with aluminum foil is suitable for processing the paper layer, however, because none of them can provide sufficient heat output within a short period of time. When the paper side is being heated by one of the known preheating units, it is necessary to run at a very high temperature to achieve a high processing output. This can lead to undesirable brown discoloration of the paper layer, especially when the thermoforming machine is being started and stopped.
When contact heating is used to increase the temperature, the temperature must remain below the melting point of the sealing lacquer, because otherwise the lacquer can be damaged. There is also the danger that particles will settle on the contact surface of the preheating unit and that these particles will then come loose during production and turn up inside the package.
The known preheating units such as contact heaters, furthermore, have a great deal of thermal inertia because of their considerable mass, and for this reason it is not possible to change the production speed quickly when such preheaters are used.